A roofing contractor is performing a reroofing project on a residential building with a complex roofline, including multiple valleys and dormers. The existing roofing material is being fully removed. To ensure proper drainage and prevent leaks in the valleys, what is the most effective underlayment and flashing application method?
- AInstall a woven shingle valley where shingles from both sides are interwoven without additional underlayment.
- BInstall a self-adhering polymer modified bitumen membrane (ice and water shield) extending at least 18 inches from the center of the valley on both sides, followed by an open or closed metal valley system.
- CInstall a single layer of 30-lb felt underlayment in the valley, centered, then cut shingles to fit.
- DApply a thick bead of asphalt mastic along the valley center directly onto the roof deck before shingle installation.
Show answer & explanationAnswer & explanation
Correct answer: B. Install a self-adhering polymer modified bitumen membrane (ice and water shield) extending at least 18 inches from the center of the valley on both sides, followed by an open or closed metal valley system.
Valleys are high-risk areas for leaks. Installing a self-adhering polymer modified bitumen membrane provides excellent waterproofing. Combining this with a properly installed open or closed metal valley system offers superior protection and durability for complex rooflines.
Why the other options are wrong
- A. Woven valleys can be effective but rely heavily on shingle integrity; a self-adhering membrane and metal system offer superior protection for complex roofs.
- C. 30-lb felt alone in a valley is insufficient for robust waterproofing, especially in high-risk areas like complex valleys.
- D. Asphalt mastic is not a primary waterproofing method for valleys and will degrade over time, leading to leaks.
Valley Underlayment & Flashing
For optimal leak protection in roof valleys, especially on complex roofs, a self-adhering polymer modified bitumen membrane combined with a metal valley system is recommended.
- Valleys are critical drainage points.
- Self-adhering membranes provide superior waterproofing.
- Metal valleys offer durability and good water shedding.
Memory trick: Valleys are weak, where water will seek; seal with sticky shield, then metal for peak.